I speak English as a first language, so I might be able to give an outsider's perspective to this thread. I always assumed non-Japanese IPs were banned from here, but apparently that's not the case.
Anyway, I speak with a Canadian accent, which is pretty homogeneous and boring. If you have any questions about how different accents sound to native speakers, feel free to ask. I don't speak Japanese, but I'll try my best with Google Translate.
My advice to anyone trying to learn English as a second language is: Go for American English unless you plan to live in Britain. Then again, English is a forgiving language, so you can pretty much choose whatever you want.
Foot-strut split isogloss Accents today are a cross between Estuary English and Received Pronunciation which both evolve over time and are becoming significantly more similar; where towards the former, accents have been strongly influenced by proximity to towns in the London Gateway involved in heavy trades, fishing and repairs and merchant sides of the Royal Navy, which was concentrated in Greenwich and Chatham. This corresponds to the name of the dialect which refers to the Thames Estuary. 'H dropping' is in general rare, as in the other Home Counties, but 't dropping' or substitution with a weaker vowel is common.[1]
However of particular note, one vowel usage is unique to West Kent which is a use of the 'a'-like 'o' for the regional (and internationally) dominant form of 'u' as in the word "strut". Technically this sound is pronounced /?/ instead of /?/, such that "cut" closely resembles "cot" and is pronounced /k??/ except before a vowel when it is /k?t/[citation needed].